The wood heads of golf clubs are prepared by machining persimmon or like wood into the specified shape and attaching a face insert to the hitting surface.
The face insert itself is made of a reinforced composite material or the like and has high durability against impact, so that no problem arises insofar as the face insert portion strikes the ball at all times. However, if repeated shots are made with the ball, especially the two-piece ball recently widely used, in contact with the clubhead except at the face insert area, the clubface becomes dented by the impact. FIG. 6 shows a dent 83 formed at a location slightly away from a face insert 81 toward the toe 82 and resulting from repetitions of impact.
To overcome the above problem, it has been practice to impregnate the wood head with a liquid resin composition comprising thermally polymerizable plastic monomers, oligomers, a mixture of such compounds or one or at least two of unsaturated polyesters and to thereafter polymerize and cure the composition by heating or by irradiation with gamma rays or the like to give the wood head improved impact resistance and durability.
In order to assure improved impact resistance and durability, it is also known to impregnate a wood head with such a resin composition, fit the impregnated head into a mold having a similar shape to the head and polymerize the composition by heating with application of pressure to obtain a compressed head.
However, these methods have the following problems.
(i) To obtain sufficient impact resistance, it is invariably necessary to impregante the wood head with an increased amount of resin composition, which renders the head heavier. It is therefore impossible to obtain a wood head having an allowable weight of up to 190 g (exclusive of the face insert and metal sole).
(ii) The heat generated by the polymerization of the resin composition produces a very high temperature in the central portion of the head to crack or deteriorate the inner portion of the head. The higher the resin impregnation ratio, the more pronounced is this phenomenon.
(iii) The wood head impregnated with the resin composition at a high ratio requires a prolonged period of time for the polymerization to completely proceed up to the central portion. The higher the impregnation ratio, the longer is the time taken.
(iv) In the case of compressed heads, the compression process breaks down wood fibers, reducing the inherent strength of wood, with the result that the junction between the head and the shaft, i.e. the neck, is liable to fracture or crack when striking golf balls.